An experimental and statistical study for design optimization of a coffee valve

Coffee is a very common beverage exported all over the world: just after roasting, coffee beans are packed in plastic or paper bags, which then experience long transfers with long storage times. Fresh roasted coffee emits large amounts of CO2 for several weeks after roasting. This gas must be gradually released, to prevent package over-inflation and to preserve aroma, moreover beans must be protected from oxygen coming from outside. For coffee correct conservation, one-way degassing valves are applied to each package: their functionality is strictly related to the interference coupling between their bodies and covers. However, these two components are usually produced by injection moulding, which makes it hard to accomplish very strict tolerances. This study, with reference to a commercial valve, investigates the distribution of the coupling diameters of bodies and covers coming from different manufacturers. The problem was approached by observing 40 pieces of each component by a high resolution profile projector. The obtained data were then the inputs for the following processing: Monte Carlo simulations were run to make a prevision of the entity of interference and of its cumulative distribution function. Four different coupling options were considered: the final results led to the determination of the best combination and of the expected defect rate (12‰), significantly lower than the actual one.